The present invention relates to mixtures of refractory metal powders usable as substrates for thin film dielectric layers formed as anodic films on the substrate material and more particularly to mixtures of tantalum and tantalum nitride powders or niobium and niobium nitride powders usable as substrate material to form high grade wet or solid electrolytic capacitors.
The use of nitrogen to improve the performance of solid electrolyte capacitors made using tantalum or niobium as the substrate material is known. U.S. Pat. No. 5,948,447, granted Sep. 5, 1995 to H. Chang/Cabot Corp., describes nitrogen doping of tantalum or niobium powder substrates to reduce leakage and speculating a beneficial effect in limiting crystal growth during anodization. The patent further describes the benefit of higher solubility of nitrogen in tantalum or niobium compared to oxygen to lower leakage by limiting movement of oxygen and a synergistic effect of controlled combinations of nitrogen and oxygen content of the tantalum or niobium substrate. T. Tripp et al/H.C. Starck, Inc. in a symposium paper have described a 30 year long effort to investigate the effects of nitrogen doping on tantalum substrates, mostly as to sputter deposited layers of tantalum or tantalum nitride but including also nitrogen doped powder and describe current work that verifies the effect of nitrogen in retarding migration of oxygen across the metal (Ta or Nb)-anodic oxide interface. In addition, capacitor manufactures believe that a large fraction of intra-agglomerate pores are necessary for making capacitors from high capacitance-voltage (CV) powders. Also, an open porosity will help to reduce the capacitor equivalent series resistance (ESR).
It is therefore an object of the present invention to provide an improved powder mixture suitable as a substrate material in high grade wet or solid electrolytic capacitors.
It is a further object of the present invention to provide a substrate that has a larger fraction of intra-agglomerate pores than the refractory metal or metal-nitride powder alone.
It is a further object of the present invention to provide a method of producing high grade electrolytic capacitors from refractory metal and metal-nitride powder mixtures.
It is a further object of the present invention to provide a thermodynamically stable substrate-anodic film interface making the system less susceptible to the degradation that occurs in the tantalum-tantalum oxide system during thermal cycling.
The objects set forth above as well as further and other objects and advantages of the present invention are achieved by the embodiments of the invention described hereinbelow.
The present invention provides an improved powder suitable as a substrate material in high grade electrolytic capacitors and method of making same. Sintered pellets made from blends of tantalum and tantalum nitride powders were found to have a higher fraction of intra-agglomerate pores than those made from tantalum or tantalum nitride alone. Pellet porosity and total intrusion volume maximizes when the Ta/TaN mixture is in the 50-75 W/W % TaN range. The total pellet pore surface area was found to be relatively independent of TaN concentration above 50% TaN. A substrate consisting of a 50/50 or 25/75 W/W % Ta/TaN mixture should produce solid capacitors with higher capacitance recovery and lower ESR. Similar results were also found for mixtures of niobium and niobium nitride powders.
The present invention uses a tantalum or niobium powder derived in various known ways and processed to have an extremely low oxygen impurity content, then introducing nitrogen in a reactor schedule that precludes re-oxidation of the refractory metal. The schedule having multiple stages of thermal processing and environmental control defined below to establish a tantalum nitride or niobium nitride powder compound without excess of nitrogen remaining and eventually cooling under inert atmosphere and air release of the powder to form only a limited oxide at room temperature.
Other objects, features and advantages of the invention will be apparent from the following description of preferred embodiments thereof, including illustrative non-limiting examples, of the practice of the process and the resulting products"" configuration, composition and performance under test conditions.